Field of the Invention
The present invention relates to a ring oscillator and a semiconductor device having the ring oscillator and particularly to a ring oscillator with a variable loop length and a semiconductor device having the ring oscillator.
Background
A ring oscillator is a circuit that generates a pulse having a given frequency through odd units of circularly connected delay circuits and is incorporated in many types of semiconductor devices. The ring oscillator is used mainly for generating a clock signal. Japanese Laid-Open Patent Publication No. 2010-117846 describes an example in which a ring oscillator is used as a random number generator.
The ring oscillator is also used for measuring the cycle of an incoming external clock signal. Clock single cycle measurement using the oscillator can be applied to a DLL (Delay Locked Loop) circuit, a DCC (Duty Cycle Corrector) circuit, etc., incorporated in a DRAM (Dynamic Random Access Memory), etc. A DRAM including a DCC circuit is disclosed in, for example, Japanese Laid-Open Patent Publication No. 2008-210436.